Corrosion-and chip-resistant coatings for high tensile steel

ABSTRACT

Corrosion- and chip-resistant coatings for high tensile steel components, such as automotive coil springs, are formed from a coating powder composition of “toughened” epoxy resin. In a single coat embodiment, the entire coating is loaded with at least 75 phr zinc powder. In a dual coat embodiment, an inner coat is loaded with at least 75 phr zinc and an outer, zinc-free coating is reinforced by the addition of fibers and/or by a foaming agent which renders it porous.

This application is a Divisional of U.S. application Ser. No.09/703,408, filed on Nov. 1, 2000.

The present invention is directed to coatings for high tensile steelobjects, such as springs formed from high tensile steel which providecorrosion and chip-resistance to the high tensile steel.

BACKGROUND OF THE INVENTION

Steel coil springs in the wheel assemblies of automobiles and othervehicles are subjected to very corrosive conditions. Conventional steelsprings were expected to rust, and to allow for such rusting,conventional coil springs were formed of appropriately thick steel.

To improve the steering and ride control of automobiles, it is desirableto move the wheels outward, toward the corners of the vehicle. Thisincreases torsional stress on the automobile body structure which mustbe nullified using a stronger frame assembly or weight reduction of thesuspension components moved toward the corners. Reduced diameter, hightensile steel, coil springs weigh less than conventional automotivesuspension coil springs, so they offer means to reduce the weight ofthese components. Super high tensile steel offers the promise of furtherweight reduction.

Herein, high tensile steel is defined as having MPa) (megapascal (N/m²)ranging from 1800 Mpa to 2100 Mpa; this includes super high tensilesteel from 1950 Mpa to 2100 Mpa or above.

High tensile steel, coil springs are scratch and notch sensitive, sothey require protection from impact damage caused by flying stones andgravel encountered during driving on paved or unpaved roads. Also, tomaintain desired metallurgical properties and prevent premature flexdamage failure, the high tensile steel cannot be heated beyond 325° F.(163° C.).

Conventional “E” coat primers and/or epoxy powder coatings used on struttype automotive suspension springs are unacceptable at inhibiting flyingstone damage as determined by low temperature gravelometer testingfollowed by accelerated scab corrosion testing of high tensile springs.Neither layer applied alone will provide the chip and corrosionprotection required.

A current approach to providing corrosion and chip resistance to hightensile steel is described in U.S. patent application Ser. No.08/728,237 filed Oct. 8, 1996, the teachings of which are incorporatedherein by reference. This patent application describes a dual layercoating including a zinc-rich, thermoset, epoxy base coat which providescorrosion-resistance and a thermoplastic outer coating which provideschip-resistance. The particular thermoplastic resin taught in U.S. Pat.No. 08/728,237 is an ethylene/acrylic acid copolymer, a relativelyexpensive material. Polyolefins can be substituted for ethylene/acrylicacid copolymer, and although polyolefins are less expensive materials,thicker layers are required to provide the requisite chip-resistance.Also, processing of thermoplastics is expensive. A general disadvantageof such a dual layer coating is that two separate coating operations arerequired and two separate heating cycles are required, the first to fuseand cure, at least in part, the epoxy resin of the base coat and asecond to fuse the thermoplastic resin of the outer coat plus completecuring of the epoxy base coat. Both the material of the base coat andthe outer coat must be cycled at a temperature below that whereat thesteel would lose its high tensile strength; accordingly, the choice ofsuitable resin systems for the two layers is limited.

It is accordingly an object of the present invention to provide coatingsfor high tensile steel which are less expensive than the coatingscurrently in use and which can be processed with a single heat cycle.

SUMMARY OF THE INVENTION

In accordance with the invention, chip- and corrosion-resistant coatingsfor high tensile steel are provided in which the resin component isentirely a “toughened” epoxy resin. Corrosion-resistance is provided byhigh zinc loading, particularly in the portion of the coating in contactwith the high tensile steel.

As one method of toughening the epoxy resin, the epoxy is adducted to(chemically bound to) between about 5 and about 25 wt % (based on totalweight of the epoxy component and the elastomer component) of anelastomer having a glass transition temperature (T_(g)) of −30° C. orbelow, preferably a T_(g) of −40° C. or below. A particularly suitableelastomeric component is carboxyl-terminated butadiene/acrylonitrile(CTBN) rubber. The elastomeric component flexibilizes the resin andremains flexible down to temperatures to which the component part may besubjected to in cold weather conditions.

In accordance with another method of toughening the epoxy resin, theepoxy resin is chemically bound as an outer shell to a soft rubber corehaving a T_(g) of about −30° C. or below, preferably about −40° C. orbelow, for example an acrylic rubber core having carboxylic acidfunctionality by which the epoxy resin of the shell is bound. Suchacrylic rubbers might be formed from (meth)acrylic acid and(meth)acrylic esters In the fused and cured coating provided by such apowder, the acrylic rubber cores are believed to act to terminate anyfracturing of the coating which may begin due to impact. In thecore/shell resins which are used to form the coating powder, the acrylicrubber comprises between about 5 and about 20 wt % of the resin based ontotal of epoxy shell and acrylic rubber core, and the epoxy of the shellthe remaining about 80 to about 95 wt %.

In accordance with another method of toughening the epoxy resin, theepoxy resin is cured with a multi-hydroxy functional curing agent havinga hydroxyl equivalent weight of at least 200, preferably at least about300, up to about 500. The distance between hydroxyl groups of such acuring agent provides flexibility to the cured epoxy resin. Thistoughening method may be used alone or in conjunction with either of thetwo above-discussed toughening methods.

Although, each of the above-described hardened epoxies exhibitchip-resistance, such chip resistance is insufficient to meet therequirements for high tensile steel springs. However, as noted above,the hardened epoxies are zinc-loaded, i.e., with at least 75 parts perhundred resin (based on total of epoxy resin, hardening resin ifpresent, and epoxy cross-linker), preferably at least about 150 phrzinc, and more preferably at least about 200 phr zinc. Surprisingly, andunexpectedly, the zinc, added for corrosion resistance, further toughensthe epoxy that is toughened by any of the three above-discussed methods.Accordingly, a toughened, zinc-loaded epoxy, at a fused and curedthickness of between about 12 and about 20 mils, preferably at leastabout 15 mils, may be applied as a single coat to high tensile steel,including high tensile steel springs, and meet present day commercialrequirements for chip- and corrosion-resistance.

A single coat of toughened, zinc-loaded epoxy has the advantage ofsimplicity, being applicable in a single powder coating operation andthen processed through a single heating cycle to heat and cure the epoxyresin.

On the other hand, a single coat of toughened, zinc-loaded epoxy iswasteful of zinc in that only the zinc in close proximity to the hightensile steel affords corrosion protection. As zinc is a relativelyexpensive material, there is provided in accordance with the invention adual coat embodiment in which an inner coat between about 1.5 and about3 mils thick is one of the toughened, zinc-loaded epoxies describedabove with respect to the single coat embodiment, and an outer coat,between about 10 and about 15 mils thick, is of the same toughened epoxyas the inner coat, but without the zinc.

As noted above, any of the above-described toughened epoxies providechip resistance, but, insufficient chip resistance to meet currentrequirements for high tensile steel springs. Accordingly, the zinc-freeouter coat is further toughened. One method of further toughening theouter layer is to add fibers to the outer coat formed of a material,such as glass, aramid or carbon, which does not melt or degrade at theprocessing temperatures to which the coating powder is subjected. Usefulfibers have diameters between about 5 and about 20 μm, and are used atbetween about 20 and about 80 phr of the resin used to form the outercoat. Another method of further toughening the outer coat is to add ablowing agent to the outer coat resin such that when processed, thedensity of the outer coat is reduced by at least about 25%, preferablyat least 40% relative to theoretical density. Above about 65% reductionin density, the coating becomes weakened. The foaming provides porosityto the outer coating. A porous outer coat, when struck by material suchas gravel may be dented, but will not fracture. Most preferably, theresin of the outer coat is further toughened by both the addition offibers as well as by foaming through use of a blowing agent.

The dual coating is by a “dry on dry” method and processed in a singleheating cycle. First, the toughened, zinc-loaded, epoxy coating powderis applied, e.g., electrostatically, to the high tensile steel, e.g., aspring, in an amount sufficient to form the inner coat of desiredthickness. Then the coating powder for the outer coat is applied in anamount sufficient to form the outer coat of desired thickness. Then thepowder coated, high tensile steel object is heated to fuse and cure thecoating powders, thereby producing the dual coating. Because the sametoughened epoxy resin is used to form both the inner and outer coats,the heating cycle produces a dual layer structure in which the twolayers are integrally bonded to each other through the cross-linking ofthe epoxy resins of the inner and outer layer.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Herein, unless otherwise noted, all percentages are by weight. In thecoating powders described herein, the epoxy resin and any tougheningresin which becomes an integral part of the cross-linked structure areconsidered to be the resin system. Amounts of other materials, such aspowdered zinc, glass, aramid or carbon fibers, blowing agents, etc. areexpressed as parts per hundred by weight relative to the resin systemcalculated as 100 parts.

Herein, standards set by General Motors are considered to be a reliable,present day standard for chip and corrosion resistance, particularlyGeneral Motor's “Chip Resistance of Coating” test (GM published test no.GM9508P) followed by GM's “Scab Corrosion Creepback Test” (GM publishedtest no. GM9511P).

The coating powder of Zn-loaded epoxy resin may be coated directly ontoa high-tensile steel component. Preferably, however, the high-tensilesteel is coated first with zinc phosphate.

The epoxy resin may be chosen from a variety of epoxy resins useful forcoating powders known in the art, such as those produced by the reactionof epichlorohydrin or polyglycidyl ether and an aromatic polyol such asbisphenol, e.g., bisphenol A. The epoxy resin should have an epoxyfunctionality greater than 1.0 and more preferably greater than 1.9.Generally the epoxy equivalent weight should be at least 170, but lowervalues may be possible in some cases; for example it may be 100 or more.Preferably the epoxy equivalent weight is less than 2300, especiallyless than 1000, for example from 130 to 1500, especially 150 to 800.Such epoxy resins may be produced, for example, by an etherificiationreaction between an aromatic or aliphatic polyol and epichlorohydrin ordichlorohydrin in the presence of an alkali such as caustic soda. Thearomatic polyol may be, for example, bis(4-hydroxyphenyl)-2,2-propane(i.e. bisphenol A), bis(4-hydroxyphenyl)-1,1-ethane,bis(4-hydroxyphenyl)-1,1-isobutane,bis(4-hydroxy-t-butylphenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane,4,4′-dihdyroxybenzophenone or 1,5-pentanediol, 1,6-hexanediol,diethylene glycol, triethylene glycol, polyethylene glycol,polypropylene glycol or dipropylene glycol, for example diglycidylethers or condensed glycidyl ethers of such diols, can be used. Otheroxirane group-containing polymers that can be used as the epoxy resin inhybrid powder coating compositions according to this invention includepolyglycidyl-functional acrylic polymers or epoxy novolak resins.Preferred epoxy resins for use in the present invention are those basedon bisphenol A.

In accordance with the first epoxy-toughening method described above,the epoxy resin, preferably a bisphenol A resin, is adducted to anelastomer having a T_(g) of −30° C. or below, preferably −40° C. orbelow. The preferred elastomer is CTBN rubber. Such epoxy/CTBN rubberadducts are described, for example, in U.K. Patent Specification1,407,851 (C. G. Taylor) published Sep. 24, 1975 and Powder Coatings.184 “Elastomer-Modified Epoxy Powder Coatings: a Review”, Apr. 13, 1994,No. 4347. To provide the necessary flexibilizing for cold-temperaturechip resistance, the CTBN component should be present at at least about5 wt % of the total of the CTBN and the epoxy components.

Above about 25 wt % CTBN, no further benefit is realized and it is notdesired to exceed 25 wt % lest there be insufficient epoxy component fora good cure. The fact that the elastomer component is chemically boundto the epoxy component, i.e., by esterification reaction of the carboxylgroups of the CTBN with epoxy groups, ensures that a complete phaseseparation does not occur during fusion and curing of the coatingpowder. However, there are microdomains of epoxy and rubber.

In the second toughening method above, a core/shell resin is used inwhich an acrylic rubber resin forms the core and the epoxy resin,preferably a bisphenol A epoxy resin, forms the shell. Again, chemicalbonding between carboxylic functionality of the acrylic rubber resin ofthe core and the epoxy resin of the shell prevents phase separationduring fusion and curing of the coating powder formed using thecore/shell resin. Such acrylic rubber modified epoxies are describe, forexample, by Dow Chemical Company in Polymer Reprints, 32 (3) pp. 358-9by H-J Sue and E. I. Garcia-Melfin.

Thermosetting epoxy resins contain either a cross-linking agent, such asa polyhydroxyl compound or a cure catalyst to effect auto-cross-linkingof the epoxy resin.

As a third toughening method, the epoxy resin is cured with apolyhydroxyl functionality having a relatively high hydroxy equivalentweight, i.e., at least about 200 up to about 500, preferably at leastabout 300. The relatively high hydroxy equivalent weight of thecross-linking agent ensures relatively long chain length between OHgroups, which chain lengths provide flexibility to the cured coating,helping to render the coatings chip-resistant. Suitable curing agentsuseful in the practice of this invention are exemplified by, but are notlimited to, phenolic curing agents, such as a bisphenol A end cappeddiglycidyl ether of bisphenol A, which is the reaction product of adiglycidyl ether of bisphenol A and bisphenol A. Examples of preferredphenolic curing agents for the epoxy resin component includes those soldunder the trademarks D.E.H.™ 87 and D.E.H.™ 85 by Dow Chemical Company,both of which are believed to be bisphenol A end capped diglycidylethers of bisphenol A. Other classes of phenolic hardeners can used aswell such as phenol- and cresol-novolac curing agents sold by GeorgiaPacific, Reichhold Chemicals and Ciba Geigy.

Zinc powder is added to provide corrosion resistance, but, as notedabove, the Zn also acts as a toughening agent in conjunction with any ofthe toughened epoxies described above. Accordingly, a single coat ofsuch Zn-rich, toughened epoxy coating can be used to form a single-coaton a high tensile steel component and provide such component with bothchip-resistance and corrosion resistance. Zinc powder useful in theinvention typically has an average particle size of about 4.0 microns.

Again, cost efficiencies are achieved by applying an inner coat, betweenabout 1.5 and about 3 mils thick, of zinc-laden, toughened epoxy incontact with the high tensile steel and applying an outer coating oftoughened epoxy which is free of zinc, but which either containsreinforcing fibers and/or is foamed through use of a foaming agent.Preferably, a zinc-free outer coating both contains fiber and is foamed.If a dual coat coating is formed, it is preferred that the resincomposition of the two coats be substantially identical, whereby agenerally continuous thermoset resin structure forms with the inner coatbeing rich in zinc and the outer coat being reinforced by fiber and/orfoaming.

If a foaming agent is used, it may be either admixed in dry form withthe coating powder, whereupon it tends to coat the surfaces of thecoating powder particles, or (preferably) is integrally incorporated inthe coating powder itself. If integrally mixed, the foaming agent mustbe activated at a temperature above that at which the coating powdercomposition is fused into a coating powder, but at or below thetemperature at which the coating powder is fused and cured to coat thehigh tensile steel. Typically, foaming agent is used at between about0.1 and about 5 phr, preferably at least about 0.5 phr, the actualamount depending upon the particular foaming agent, the particular resinsystem, the processing conditions and the degree to which densityreduction is desired. Depending upon the amound of ingredients inaddition to the resin, the foaming agent is used at between about 0.1and about 3 wt % of the entire formulation, preferably at least about0.3 wt %.

The foaming coating powders used in the present invention may be used bythemselves to coat substrates. Generally foaming in coating powders hasbeen considered desirable. Thus it has been a general object of coatingpowders to avoid out-gassing, e.g., out-gassing of water, which producepinholes and detract from the appearance of the coating. However, inaccordance with one aspect of the present invention, functionaladvantages are found for coating powders which produce a foamed coating.While a foamed coating is described above as a top coat in respect for adual-coating for high tensile steel, a foamed coating may be used byitself for impact resistance where the requirements are less stringent.

Foamed coatings also provide both thermal and acoustical insulation.Thus, for example, a foamed coating might be used to coat an automotiveoil pan or on the interior of automotive door panels to damp vibration.

Foamed coatings used in the present invention are not resilient, butrather indent or crush when impacted. One application for such acrushable foamed coating is in the area of providing the appearance ofvery minimal tolerance between component parts where manufacturingrequirements allow for a greater tolerance. For example, foamed coatingson the door and door jam of a hotel safe will crush to form theappearance of an extremely tight fit even if the actual tolerance isgreater.

A blowing or foaming agent to be incorporated in a coating powder mustbe selected so as not to foam during the heat fusion of the materials toform the coating powder, typically at a temperature of between about180° C. and about 240° C. for epoxy coating powders, but producesignificant gas for foaming at the fusion/curing temperature of thecoating powder, typically about 300° C. or above for epoxy coatingpowders. A currently preferred heat-activated foaming agent for epoxycoatings is p-toluene sulfonyl hydrazide, such as that sold as Celogen®TSH sold by Uniroyal Chemical Company. Other suitable blowing agentsinclude, but are not limited to 2,2′-azobisisobutyro nitrile,dinitrosopentamethylenetetramine, sodium bicarbonate, ammoniumcarbonate, silicon oxyhydride and azocarbonamide.

Chemical foaming agents such as sodium borohydride might also be used toproduce foaming. To foam, sodium borohydride requires a proton donor,such as water. U.S. Pat. No. 4,491,554, the teachings of which areincorporated herein by reference, describes the use in plastics of salthydrates in conjunction with alkali metal borohydrides to producefoaming at elevated temperature, the salt hydrate only releasing waterabove a certain threshold temperature. For epoxy-based coating powders,the combination of sodium borohydride and alumina trihydrate is a usefulcombination of foaming agent and proton donor. As described in U.S. Pat.No. 4,491,554, alumina trihydrate releases no water below 260° C. butreleases significant amounts of water at temperatures above 300° C.Thus, the coating powder can be compounded at temperatures below 260° C.and fused/cured at temperatures above 300° C. so as to produce a foamedcoating.

The concept of foamed coating powders is not limited to epoxy-basedcoating powders, but applies to all types of coating powders, including,but not limited to acrylic coating powders, polyester coating powders,silicone-based coating powders, etc. The choice of foaming agents forsuch coating powders will be selected according to the differences intemperatures at which such coating powders are compounded and a thetemperatures at which such coating powders are fused and cured.

An alternative way of producing a foamed coating in which two chemicalcomponents are required to produce gas is to prepare a dry blend of twocoating powders, one of which contains one of the chemical componentsand the other of which contains the other chemical component. Forexample, an epoxy coating powder containing sodium borohydride might bedry-blended with an acrylic coating powder containing carboxylic acidfunctional acrylic polymers. When fused and cured, the carboxylic acidfunctionality of the acrylic coating powder will contribute the protonsfor foaming the sodium borohydride.

The coatings powders useful in the invention may also incorporate minorcomponents known in the art, e.g., pigments, flow control agents, etc.

Coating powders used to provide the chip-resistant andcorrosion-resistant coatings of the present invention are produced inthe usual manner. The components are blended, and then aremelt-compounded with heating above the melting point of the resin for ashort time, e.g., 30-90 sec., so that no significant curing occurs. Themolten compound is extruded, and after extrusion, the composition israpidly cooled. The composition is then ground and, as necessary, theparticulates sorted according to size. For electrostatic coating, theparticles are generally in the 5-100 micron size range with a majorportion generally being in the 20-40 micron size range. Largerparticulates are useful for fluidized bed coating operations.

The invention will now be described in greater detail by way of specificexamples.

EXAMPLE 1

A coating powder is formed from the following ingredients:

Components PHR ¹Araldite ® GT 7074 92 ²Araldite ® GT 7226 8 ³D.E.H. ™ 8720.8 ⁴Epon ™ P-101 3 ⁵Zinc Dust 64 250 ⁶Raven 1255 3 Hand blend, extrudeinto sheets, air cool, break into chips, then add ⁷Cab-O-Sil M5 1.1Grind at high speed to powder and screen at 100 mesh. ¹Araldite ® GT7074 is a diglycidyl ether of bisphenol A epoxy resin with a weight perepoxide between 935 and 1175 that is commercially available fromCiba-Geigy Corporation. ²Araldite ® GT 7226 is a master batch epoxyresin containing 90 wt % of a diglycidyl ether of bisphenol A epoxyresin with a weight per epoxide between 795 and 895 and 10 wt % ofAcronal ® 4F acrylic flow modifier. Master batch is commerciallyavailable from Ciba-Geigy Corporation. ³D.E.H. ™ 87 is a bisphenol A endcapped diglycidyl ether of bisphenol A that has an hydroxyl equivalentweight between 370 and 400 and is commercially available from DowChemical Company. ⁴Epon Curing Agent ® P-101 is an imidazole adduct witha diglycidyl ether of bisphenol A epoxy resin that is commerciallyavailable from Shell Chemical Company. ⁵Zinc Dust 64 is a zinc powderthat is commercially available through E. W. Kaufmann Corporation.⁶Raven 1255 is a carbon black pigment that is commercially availablefrom Columbian Chemical. ⁷Cab-O-Sil ® M5 is a fumed silica that iscommercially available from Cabot Corporation.

The epoxy powder coating composition in example 1 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 2

A coating powder is formed from the following ingredients:

Components PHR ¹RSS-1704 100 ²Casamine OTB 3.7 ³MB 2MI 0.15 ⁴Resiflow ®P-67 1.4 Zinc Dust 64 250 Raven 1255 4 Hand blend, extrude into sheets,air cool, break into chips, then add Cab-O-Sil M5 1.1 Grind at highspeed to powder and screen at 100 mesh. ¹RSS-1704 is an experimentalCTBN (carboxy terminated butadiene/acrylonitrile) modified diglycidylether of bisphenol A epoxy resin with a weight per epoxide between 850and 1050 that is available from Shell Chemical Company. ²Casamine OTB isan ortho tolyl biguanide curing agent and is commercially available fromThomas Swan & Company. ³MB 2 MI is a master batch containing 95 wt % ofDyhard MI, 2-methyl imidazole, and 5 wt % Cab-O-Sil M5 and iscommercially available from SKW Chemicals, Inc. ⁴Resiflow ® P-67 is anacrylic flow aid that is a polyacrylate and is commercially availablefrom Estron Chemical Company.

The epoxy powder coating composition in example 2 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 3

A coating powder is formed from the following ingredients:

Components PHR ¹Toughener 100 Casamine OTB 4.84 MB 2MI 0.20 Resiflow ®P-67 1.4 Zinc Dust 64 250 Raven 1255 4 Hand blend, extrude into sheets,air cool, break into chips, then add ²Aluminum Oxide C 0.7 Grind at highspeed to powder and screen at 100 mesh. ¹The toughener is a core/shellresin in which the core is an acid functional, low T_(g) acrylic rubberand the shell an epoxy resin. ²Aluminum Oxide C is a fumed alumina thatis commercially available from Sullivan Associates, Inc.

The epoxy powder coating composition in example 3 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 4 (COMPARATIVE)

A coating powder is formed from the following ingredients:

Components PHR Araldite ® GT 7074 92 Araldite ® GT 7226 8 D.E.H. ™ 8720.8 Epon ™ P-101 3 ¹Texaquart 900 1 Raven 1255 3 Cab-O-Sil M5 0.38 Handblend, extrude into sheets, air cool, break into chips, then addCab-O-Sil M5 2.5 Grind at high speed to powder and screen at 100 mesh.¹Texaquart 900 is a complex fatty acid that is commercially availablethrough E. W. Kaufmann Corporation.

The epoxy powder coating composition in example 4 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 5

A coating powder is formed from the following ingredients:

Components PHR ¹KR-102 100 Casamine OTB 2.92 MB 2MI 0.12 Resiflow ® P-671.4 Zinc Dust 64 250 Raven 1255 4 Hand blend, extrude into sheets, aircool, break into chips, then add Aluminum Oxide C 0.7 Grind at highspeed to powder and screen at 100 mesh. ¹KR-102 is a CTBN (carboxyterminated butadiene/acrylonitrile) modified diglycidyl ether ofbisphenol A epoxy resin with a weight per epoxide between 1,100 and1,300 that is commercially available from Kukdo Chemical Comopany anddistributed by GCA Chemical Company.

The epoxy powder coating composition in example 4 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 6 (comparative)

A coating powder is formed from the following ingredients:

Components PHR KR-102 100 Casamine OTB 2.92 MB 2MI 0.12 Resiflow ® P-671.4 Raven 1255 4 Hand blend, extrude into sheets, air cool, break intochips, then add Aluminum Oxide C 0.7 Grind at high speed to powder andscreen at 100 mesh.

The epoxy powder coating composition in example 4 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 7 (comparative)

A coating powder is formed from the following ingredients:

Components PHR KR-102 100 Casamine OTB 2.92 MB 2MI 0.12 Resiflow ® P-671.4 ¹Suzorite 325-HK mica 50 Raven 1255 4 Hand blend, extrude intosheets, air cool, break into chips, then add Aluminum Oxide C 0.7 Grindat high speed to powder and screen at 100 mesh. ¹Suzorite 325-HK Mica isphilogopite mica powder that is commercially available through SuzoriteMica Products.

The epoxy powder coating composition in example 6 was thenelectrostatically spayed onto high tensile steel springs at variousthickness.

EXAMPLE 8 (Corvel® 13-7004) (comparative)

A coating powder is formed from the following ingredients:

Components PHR ¹D.E.R. ™ 664U 84 ²Araldite ® 9496 16 ³Casamine OTB 5 ⁴MB2MI 0.2 Zinc Dust 64 250 ⁵Tioxide TR93 40 ⁶BK-5099 0.26 Texaquart 9003.0 Hand blend, extrude into sheets, air cool, break into chips, thenadd ⁷Aluminum Oxide C 0.79 Grind at high speed to powder and screen at100 mesh.

TABLE 1 Test Results (Example 1) Chip resistance pass @ (15 mils (per GM9508P-Method B) fail @ < 15 mils Scab corrosion (per GM 9511P) pass withno peelback Environmental corrosion cycle J pass (per GM9505P) Results(Example 2) Chip resistance pass @ (11 mils (per GM 9508P-Method B) fail@ < 11 mils Scab corrosion (per GM 9511P) pass with no peelback Results(Example 4) Chip resistance fail @ 15-21 mils (per GM 9508P-Method B)This shows that without zinc, the coating fails. Results (Example 5)Chip resistance pass @ (16 mils (per GM 9508P-Method B) fail @ < 15 milsScab corrosion (per GM 9511P) pass with no peelback Results (Example 6)Chip resistance fail @ 16-18 mils (per GM 9508P-Method B) fail @ 21-24mils This shows that without zinc the CTBN modified epoxy fails. Results(Example 7) Chip resistance fail @ 15-18 mils (per GM 9508P-Method B)fail @ 20-23 mils This shows that a CTBN modified epoxy, with a hardmica filler, but no zinc, fails. Results (Corvel ® 13-7004) (Example 8)Chip resistance fail @ 2-5 mils (per GM 9508P-Method B) fail @ 10-15mils fail @ 20-23 mils Scab corrosion (per GM 9511P) pass with nopeelback This shows that a zinc loaded single coat without tougheneningfails.

EXAMPLE 9

Base coat 9

A coating powder is formed from the following ingredients:

Components PHR ¹Araldite ® GT 7074 92 ²Araldite ® GT 7226 8 ³D.E.H. ™ 8720.8 ⁴Epon ™ P-101 3 ⁵Zinc Dust 64 250 ⁶Raven 1255 3 Hand blend, extrudeinto sheets, air cool, break into chips, then add ⁷Cab-O-Sil M5 1.1Grind at high speed to powder and screen at 100 mesh.

Top coat 9

A coating powder is formed from the following ingredients:

Components PHR Araldite ® GT 7074 92 Araldite ® GT 7226 8 D.E.H. ™ 8720.8 Epon ™ P-101 3 ⁸737 BC 50 Raven 1255 2 Cab-O-Sil M5 2.5 ⁹Texaquart900 3.0 Hand blend, extrude into sheets, air cool, break into chips,then add Cab-O-Sil M5 0.36 Grind at high speed to powder and screen at100 mesh. ¹Araldite ® GT 7074 is a diglycidyl ether of bisphenol A epoxyresin with a weight per epoxide between 935 and 1175 that iscommercially available from Ciba-Geigy Corporation. ²Araldite ® GT 7226is a master batch epoxy resin containing 90 wt % of a diglycidyl etherof bisphenol A epoxy resin with a weight per epoxide between 795 and 895and 10 wt % of Acronal ® 4F acrylic flow modifier. Master batch iscommercially available from Ciba-Geigy Corporation. ³D.E.H. ™ 87 is abisphenol A end capped diglycidyl ether of bisphenol A that has anhydroxyl equivalent weight between 370 and 400 and is commerciallyavailable from Dow Chemical Company. ⁴Epon Curing Agent ® P-101 is animidazole adduct with a diglycidyl ether of bisphenol A epoxy resin thatis commercially available from Shell Chemical Company. ⁵Zinc Dust 64 isa zinc powder that is commercially available through E. W. KaufmannCorporation. ⁶Raven 1255 is a carbon black pigment that is commerciallyavailable from Columbian Chemical. ⁷Cab-O-Sil ® M5 is a fumed silicathat is commercially available from Cabot Corporation. ⁸737 BC is millede-glass fibers that are commercially available from Owens Coming.⁹Texaquart 900 is a complex fatty ester for antistatic enhancement andis commercially available from E. W. Kaufmann Corporation.

The base powder coating composition in example 9 was thenelectrostatically spayed onto high tensile steel, followed byapplication of the top coat at various thickness.

EXAMPLE 10

Base coat 9 (as in Example 9)

Top coat 10

A coating powder is formed from the following ingredients:

Components PHR Araldite ® GT 7074 92 Araldite ® GT 7226 8 D.E.H. ™ 8720.8 Epon ™ P-101 3 737 BC 50 Raven 1255 2 Cab-O-Sil M5 2.5 Texaquart900 3.0 Hand blend, extrude into sheets, air cool, break into chips,then add Cab-O-Sil M5 0.36 Grind at high speed to powder and screen at100 mesh, then add ¹Celogen TSH 1.8 Henschel blend at high speed andscreen at 100 mesh. ¹Celogen TSH is a heat actuated foaming agent,p-toluene sulfonyl hydrazide, that is commercially available fromUniroyal Chemical Company.

The base powder coating composition in example 10 was thenelectrostatically spayed onto high tensile steel, followed byapplication of the top coat at various thickness.

EXAMPLE 11

Base coat 9 (as in Example 9)

Top coat 11

A coating powder is formed from the following ingredients:

Components PHR Araldite ® GT 7074 92 Araldite ® GT 7226 8 D.E.H. ™ 8720.8 Epon ™ P-101 3 Raven 1255 2 Cab-O-Sil M5 2.5 Texaquart 900 3.0 Handblend, extrude into sheets, air cool, break into chips, then addCab-O-Sil M5 0.36 Grind at high speed to powder and screen at 100 mesh,then add Celogen TSH 1.3 Henschel blend at high speed and screen at 100mesh.

The base powder coating composition in example 11 was thenelectrostatically spayed onto high tensile steel, followed byapplication of the top coat at various thickness.

TABLE 2 Test Results (Example 8) Chip resistance pass @ ≧ 16 total mils(per GM 9508P-Method B) fail @ < 15 total mils Scab corrosion (per GM9511P) pass with no peelback Test Results (Example 9) Chip resistancepass @ ≧ 15 total mils (per GM 9508P-Method B) fail @ < 15 total milsScab corrosion (per GM 9511P) pass with no peelback Environmentalcorrosion cycle J (per GM9505P) pass Test Results (Example 10) Chipresistance pass @ ≧ 17 total mils (per GM 9508P-Method B) fail @ < 17total mils Scab corrosion (per GM 9511P) pass with no peelbackEnvironmental corrosion cycle J pass (per GM9505P) Test Results (Example11) Chip resistance pass @ ≧ 15 total mils (per GM 9508P-Method B) fall@ < 13 total mils Scab corrosion (per GM 9511P) pass with no peelbackEnvironmental corrosion cycle J

What is claimed is:
 1. A coating powder comprising a toughenedthermosetting epoxy resin, from 20 to 80 phr of fibers, and a sufficientamount of a p-toluene sulfonyl hydrazide blowing or foaming agent suchthat when the coating powder is applied to a substrate and fused andcured, the resulting coating has a density reduced at least about 25%relative to the theoretical density of the resulting coating in theabsence of p-toluene sulfonyl hydrazide.
 2. The coating powder of claim1 having sufficient p-toluene sulfonyl hydrazide such that when thecoating powder is applied to a substrate and fused and cured, theresulting coating has a density reduced at least about 40% relative tothe theoretical density of the resulting coating in the absence ofp-toluene sulfonyl hydrazide.
 3. The coating powder of claim 1, whereinsaid fibers have a diameter of from 5 μm to 20 μm.
 4. A coating on asubstrate formed by fusing and curing the coating powder of claim 1,said coating having a density reduced at least about 25% relative to thetheoretical density of the same coating formed in the absence of saidp-toluene sulfonyl hydrazide.
 5. A coating powder comprising a toughenedthermosetting epoxy ran, from 20 to 80 phr of fibers, and a sufficientamount of an alkali metal borohydride blowing or foaming agent such thatwhen the coating powder is applied to a substrate and fused and cured inthe presence of a proton donor, the resulting coating has a densityreduced at least about 25% relative to theoretical density of theresulting coating in the absence of an alkali metal borohydride.
 6. Thecoating powder of claim 5 wherein said proton donor is contained withinsaid coating powder.
 7. The coating powder of claim 6 wherein saidproton donor is alumina trihydrate.
 8. The coating powder of claim 5having sufficient alkali metal borohydride such that when the coatingpowder is applied to a substrate and fused and cured in the presence ofa proton donor, the resulting coating has a density reduced at leastabout 40% relative to theoretical density of the resulting coating inthe absence of an alkali metal borohydride.
 9. The coating powder ofclaim 5, wherein said fibers have a diameter of from 5 μm to 20 μm. 10.A coating on a substrate formed by fusing and curing the coating powderof claim 5, said coating having a density reduced at least about 25%relative to the theoretical density of the resulting coating in theabsence of an alkali metal borohydride.
 11. A method of coating asubstrate comprising: applying to a substrate a coating powdercomprising 100 weight parts of a curable epoxy resin and from 75 partsper hundred resin (phr) to 250 phr of zinc powder to form a coatinglayer; applying to said coating layer the coating powder of claim 1 toform a dual coating layer; and heating to fuse and cure said dualcoating layer.
 12. A method of coating a substrate comprising: applyingto a substrate a coating powder comprising 100 weight parts of a curableepoxy resin and from 75 parts per hundred resin (phr) to 250 phr of zincpowder to form a coating layer; applying to said coating layer thecoating powder of claim 5 to form a dual coating layer; and heating tofuse and cure said dual coating layer.